In Non-Patent Document 1, high electron mobility transistors (HEMT) are disclosed. The high electron mobility transistors have an AlGaN/GaN heterostructure epitaxially grown on a sapphire substrate. In order to manufacture the high electron mobility transistors, after forming a low-temperature GaN layer on the sapphire substrate, an i-type GaN layer of 2 to 3 μm is formed. On the GaN layer, an i-type AlGaN layer of 7 nm, an n-type AlGaN layer of 15 nm, and an i-type AlGaN layer of 3 nm are formed in that order. The Schottky electrode is composed of Ni(3 nm)/Pt(300 nm)/Au(300 nm).
Non-Patent Document 1: “Improvement of DC Characteristics of AlGaN/GaN High Electron Mobility Transistors Thermally Annealed Ni/Pt/Au Schottky Gate” Japanese Journal of Applied Physics Vol. 43, No. 4B, 2004, pp. 1925-1929.